The activity or concentration of a specific target ion in a measured medium is an important measured variable in environmental analytics and in a number of chemical or biochemical methods in the laboratory or in industrial processes. To a first approximation, the activity of a target ion can be set equal to the concentration of the target ion in dilute solutions.
A special case of activity measurement or concentration measurement of ions is the measurement of the pH value or pOH value of a measured medium. The pH value corresponds to the negative base 10 logarithm of the H+ ion activity in the measured medium, which can be set to equal the H+ ion concentration in dilute solutions. To a first approximation, the H+ ion activity can be set equal to the H+ ion concentration in dilute solutions. Analogous to the pH value, the pOH value is defined as the negative base 10 logarithm of the OH− ion activity or the OH− ion concentration to a good approximation for dilute solutions. The two values are related by the constant ionic product of water:pH+pOH=14.
Starting with the pH value or the pOH value, the associated H+ ion or OH− ion activities and/or the corresponding concentrations can thus be ascertained.
A measuring transducer for determining an activity of a target ion, also called an ion selective measuring transducer or an ion selective electrode in the following, includes, as a rule, a measuring half cell with an ion selective element, for example, an ion selective, solid, or polymer, membrane. The relative change of the equilibrium Galvani voltage between a measured medium and a potential sensing electrode is basically effected predominantly by the change of the activity of a specific target ion. Based on a reference potential of a basically constant potential, reference half cell, for example, a reference electrode of the second type such as the Ag/AgCl reference electrode, the activity of the target ion in the measured medium can be determined with little effort by means of a high impedance, highly accurate, voltmeter. Serving as measurement signal of such an ion selective measuring transducer for representing the activity of the target ion is thus the potential difference between the measuring half cell and the reference half cell. Ion selective electrodes are described, for example, in “Ion-selective electrodes”, J. Koryta and K. Stulik, Cambridge University Press, 1983, Pg. 61 or in “Das Arbeiten mit ionenselektiven Elektroden”, (“Working with Ion Selective Electrodes”) K. Cammann, H. Galster, Springer, 1996.
The best known and most frequently applied ion selective measuring transducer is the pH glass electrode. The measuring half cell of the glass electrode has, as a rule, a tubular glass housing closed on one end by a membrane comprising a pH sensitive glass. The tubular glass housing is filled with an inner electrolyte, for example, a buffer solution containing chloride, and a potential sensing element, for example, a chloridized silver wire, extends into the buffer solution. A measuring half cell potential dependent on the pH value forms at the glass membrane in contact with the measured medium. As a rule, a reference electrode of second type, for example, an Ag/AgCl electrode or a calomel electrode, serves as reference half cell, which has a liquid junction between a half cell space containing the reference electrolytes and the measured medium. The potential difference between the measuring half cell potential, which can be tapped at the potential sensing element of the measuring half cell, and the reference potential (which is ideally independent of the pH value of the measured medium) of the reference half cell forms the measurement signal of the measuring transducer and is a direct measure for the H+ ion activity or the pH value of the measured medium.
Although such potentiometric measuring transducers assure very precise and reliable measurement results and are well established both in the laboratory as well as in process analytics, they have a number of disadvantages. For example, defects or degradation phenomena can occur, as a rule, in the reference electrodes of second type serving as a reference half cell, degrading the quality of the measurement. For example, the inner electrolyte of the reference half cell can leak out or dry out; the liquid junction, via which the reference half cell of second type is in contact with the measured medium, can become clogged by solids, especially difficultly soluble salts; or electrode poisons can penetrate into the reference half cell via the liquid junction. In general, the potential of such reference half cells tends in practice to drift, i.e. undergo a slow, but steady change of the reference potential. Diffusion potentials and streaming potentials can also contribute to error.
In pH selective measuring transducers, which are embodied as glass electrodes, the very thin pH sensitive glass membrane is complex to manufacture and extraordinarily sensitive to handling. Breaking of the glass membrane can lead to shards, which can get into the measured medium. If the measured medium is, for example, a product manufactured in a pharmaceutical or food technical process or an intermediate product, in the case of such a glass fracture, the measured medium must be discarded, in order to prevent an endangering of the end consumer by shards in the product.
Due to the low conductivity of the pH sensitive glass membrane, it is additionally required to measure the potential difference between the leads of the measuring transducer at a very high impedance. This situation can lead to instabilities in the measurement and measured value corruptions. Due to the instances of high resistivity of the glass forming the glass membrane, there are limits to the miniaturization of the pH glass electrode, since in decreasing the glass membrane area, the resistance of the measuring half cell always becomes greater. Therefore, there has long been the need for an alternative measuring method with more robust measuring transducers for determining the pH, or pOH, value.